fix: reduce background opacity to 50% for subtler appearance

Changed background from bg-card to bg-card/50 for both TrackControls
and MasterControls to make the border more prominent.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

components/controls/MasterControls.tsx

@@ -36,7 +36,7 @@ export function MasterControls({
 }: MasterControlsProps) {
   return (
     <div className={cn(
-      'flex flex-col items-center gap-3 px-4 py-3 bg-muted/20 border-2 border-accent/50 rounded-lg',
+      'flex flex-col items-center gap-3 px-4 py-3 bg-card/50 border-2 border-accent/50 rounded-lg',
       className
     )}>
       {/* Master Label */}

components/tracks/TrackControls.tsx

@@ -57,7 +57,7 @@ export function TrackControls({
 }: TrackControlsProps) {
   return (
     <div className={cn(
-      'flex flex-col items-center justify-between h-full px-3 py-2 bg-muted/20 border-2 border-accent/50 rounded-lg',
+      'flex flex-col items-center justify-between h-full px-3 py-2 bg-card/50 border-2 border-accent/50 rounded-lg',
       className
     )}>
       {/* Pan Control - Top */}